Integrins are heterodimeric Type I transmembrane proteins formed of two subunits (one alpha subunit and one beta subunit), and mediate many different cell-cell and cell-extracellular matrix interactions. Functionally, integrins have been shown to be involved in diverse biological processes, including leukocyte migration and recirculation and the immune response. In mammals, there are 18 known alpha subunits and eight known beta subunits, which combine to form 24 distinct integrins. Ligand specificity is determined in large part by the particular combinations of alpha and beta subunits expressed, while affinity for ligand is modulated by integrin conformational changes and is divalent-cation dependent.
The ligands for integrins form a structurally diverse group that includes extracellular matrix proteins such as collagens, fibronection, vitronectin and laminins; counter-receptors such as the cellular adhesion molecules (for example, vascular cellular adhesion molecule or VCAM), and plasma proteins. Numerous pathogenic microorganisms also utilize integrins to initiate infection or as sites for toxin binding. The structurally diverse ligands share an exposed glutamic or aspartic acid residue, usually present in an extended, flexible loop, which is important for recognition by integrins.
The alpha4 integrins (alpha 4 partnered with either the beta1 or beta7 subunit) play an important role in the immune system. Alpha4beta1 is expressed on lymphocytes and myeloid cells; it appears to be the major binding partner for vascular cell adhesion molecule (VCAM). VCAM is ubiquitously expressed on vascular endothelium, is up regulated during inflammation, and binds alpha4beta7 as well as alpha4beta1 (albeit weakly to alpha4beta7). Though also detected on d peripheral T cells, B cells, NK cells and eosinphils, alpha4beta7 is most highly expressed on a subpopulation of CD4+CD45RA-memory T cells which has been shown to preferentially home to the gut. The primary ligand for the alpha4beta7 heterodimer is mucosal addressin cell adhesion molecule 1 (MAdCAM-1 or MAdCAM), which is expressed in gut endothelium.
In addition to pairing with the alpha4 chain, the beta7 subunit also partners with alphaE to form alphaEbeta7, which is primarily expressed on intraepithelial lymphocytes (IEL) in intestine, lung and genitourinary tract. AlphaEbeta7 is also expressed on dendritic cells in the gut. The alphaEbeta7 heterodimer binds to E-cadherin, which is expressed on epithelial cells. The IEL cells are thought to provide a mechanism for immunosurveillance within the epithelial compartment.
Antibodies that bind alpha4 and inhibit binding of alpha4beta1 to VCAM-1 and fibronection mapped to a 52-amino acid region of alpha4, between residues 152 and 203 (Schiffer et al., J. Biol. Chem. 270:14270; 1995). Tidswell et al. (J. Immuno 159:1497; 1997) identified domains of beta7 that are important in binding to MAdCAM-1, utilizing a panel of antibodies that bind beta7 in a mouse/human chimeric beta7 subunit approach. They found that six of seven antibodies that inhibited binding to MAdCAM-1 and E-cadherin mapped to a region comprising amino acids 176 through 250, which appears to have homology to the metal-ion dependent adhesion site (MIDAS) of other integrin subunits. One of the antibodies used by Tidswell et al. was an alpha4beta7 heterodimer specific antibody referred to as ACT-1.
The ACT-1 antibody was originally described by Lazarovitz et al. (J. Immunol. 133:1857; 1984) as an antibody developed by immunizing mice with human tetanus toxoid-specific T lymphocyte line from PBMC. Later it was shown that ACT-1 binds to the alpha4beta7 heterodimer specifically (Schweighoffer et al., J. Immunol. 151:717, 1993). While ACT-1 does not bind murine alpha4beta7, it does bind alpha4beta7 from least some non-human primate species, and has been shown to attenuate spontaneous colitis in captive cotton-top tamarins (Hesterberg et al., Gastroenterology 111:1373; 1996)
ACT-1 has been humanized and evaluated as a human therapeutic in ulcerative colitis (Feagan et al., N Engl J Med. 352:2499; 2005), and recently in Crohn's disease (Feagan et al, Clinical Gastroenterology and Hepatology, 6:1370, 2008). Humanized ACT-1, also known as vedolizumab, is described in WO 98/06248 and US patent 7,147,85, as well as WO 07/061,679 and US 2007-0122404. Another humanized antibody, natalizumab (Tysabri®), has been used to treat Crohn's disease. Natalizumab is a humanized version of an alpha4-specific murine antibody. Vedolizumab has been shown to lead to a neutralizing anti-humanized antibody response in a portion of patients, and natalizumab has been associated with progressive multifocal leukoencephalopathy (PML), a neurological disorder that is associated with reactivation of prior infection with JC virus in immunocompromised individuals. Accordingly, there is a need for a therapeutic agent that ameliorates these disadvantages while disrupting the alpha4beta7/MAdCAM-1 pathway.